The present invention relates to a process for producing anatase titanium oxide particularly having high photocatalytic activity as one of photocatalysts used, for example, as environment cleaning materials, for example, for the removal of harmful materials, the deodorisation/decomposition of offensive odor substances, antifouling, and sterilization, and more particularly to a process for producing anatase titanium oxide powder having high photocatalytic activity and large specific surface area, and a process for producing anatase titanium oxide slurry, containing the anatase titanium oxide powder, with excellent stability and dispersibility.
The present invention also relates to a process for producing a coating material of titanium oxide as one of photocatalysts used, for example, as environment cleaning materials, for example, for the removal of harmful materials, the deodorisation/decomposition of offensive odor substances, antifouling, and sterilization. More particularly, the present invention relates to a process for producing a titanium oxide coating material which, particularly when anatase titanium oxide having high photocatalytic activity is coated at a low temperature, the photocatalytic activity can persist for a long period of time, and, at the same time, can maintain high adhesion strength between the coating and the member as the substrate for a long period of time, does not deteriorate the appearance of the member by virtue of excellent transparency of the coating, and enables the production of a binder for improving adhesion strength.
Photocatalysts are materials that, through radicals (hydroxy radicals, superoxide anions, etc.) generated upon the application of ultraviolet light onto the surface thereto, can function to perform, for example, the removal of harmful materials (for example, benzene, dioxin, and volatile organic compounds), th deodorisation/decomposition of offensive odor substances (substanc s regulated by the Offensive Odor Control Law), antifouling, and sterilization.
In recent years, an attempt to utilize the above functions by coating the photocatalyst onto the surface of objects has been made. A number of oxides can be utilized as photocatalysts. Among them, titanium oxide is in many cases used as one of the photocatalysts, and, among others, anatase titanium oxide is superior from the viewpoints of both the function and safety.
Specifically, titanium oxide is classified into three crystal forms, i.e., anatase, rutile and hrookite forms, and amorphous form. Among them, anatase titanium oxide has the highest photocatalytic activity.
Anatase titanium oxide powder can be produced by a gas phase process and a liquid phase process. For each process, conventional techniques will be described.
Degussa P-25 manufactured by Nippon Aerosil Co., Ltd. is representative anatase titanium oxide produced by the gas phase process. In this process, titanium oxide powder having a specific surface area of 40 m2/g (BET method) can be produced by hydrolysis of titanium chloride in an oxygen atmosphere at a high temperature of 1000xc2x0 C.
Further, there is also a report such that anatase titanium oxide is produced by a CVD (chemical vapor deposition) process at a controlled furnace temperature of 600 to 800xc2x0 C. (Kagaku Kogaku Ronbunshu (J. Chem. Eng. Japan), Vol. 16, No. 3, 584-587, May 1990).
Sol-gel process, HyCOM (hydrothermal crystallization in Organic Media), and sulfuric acid process have been proposed for the production of anatase titanium oxide by the liquid phase process.
According to the sol-gel process, titanium oxide is produced from an alkoxide in the same manner as in the production of silica, and the sol-gel process should involve two steps, i.e., the step of preparing titanium hydroxide by hydrolysis and the step of sintering wherein titanium hydroxide is polycondensed by heating to give titanium oxide. Further, both the steps are carried out under the atmospheric pressure (for th sol-gel process, see, for example, xe2x80x9cThe Science of Sol-Gel Method,xe2x80x9d 8-15, published in July 1988 by AGNE SHOFU PUBLISHING INC.)
When anatase titanium oxide is prepared by the sol-gel process, the above step of sintering is indispensable, and the heating temperature for sintering should be in the range of 300 to 700xc2x0 C. The reason why the heat treatment in the specific temperature range is necessary is as follows. When the heat treatment is carried out at a temperature below 300xc2x0 C., titanium oxide remains unchanged from the amorphous form. On the other hand, when the heat treatment is carried out at a temperature above 700xc2x0 C., anatase titanium oxide is converted to a crystal form having lower photocatalytic activity, i.e., rutile form.
In HyCOM, moisture contained in gas or water vapor produced from a separate water tank is fed, as water necessary for hydrolysis of an alkoxide, into a solvent with the titanium alkoxide being dissolved therein by the application of pressure (10 kg/cm2G), thereby producing titanium oxide. In this case, the solvent with the alkoxide being dissolved therein and water are placed separately form each other in the apparatus. That is, water is absent in the starting material.
Titanium oxide produced by HyCOM is highly heat-resistant anatase titanium oxide which, for example, even after baking at 900xc2x0 C., maintains the anatase form and has a specific surface area of 40 m2/g (J. Mater. Sci. Lett., 15, 197 (1996)).
As described in Japanese Patent Laid-Open No. 171408/1995, in the sulfuric acid process, an acidic titanium sol, prepared by heating and hydrolyzing titanium sulfate, is adjusted to pH 7 by the addition of sodium hydroxide, and filtration and washing are then carried out to prepare a crystal. Subsequently, water is added to the resultant titanium oxide wet cake to prepare titanium oxide slurry. The titanium oxide slurry is adjusted to pH 7 by the addition of sodium hydroxide, followed by hydrothermal treatment in an autoclave at 150xc2x0 C. for 3 hr. Thereafter, the hydrothermally treated slurry is adjusted to pH 7 by the addition of nitric acid, and is then filtered. The cake is then washed with water, and is dried (110xc2x0 C., 3 hr) to prepare titanium oxide.
Next, conv ntional production processes of titanium oxide-containing liquid and slurry will be d scribed.
Japanese Patent Laid-Open No. 99041/1996 proposes a production process which comprises the steps of: adding polyethylene glycol or ethylene oxide to a titania sol, prepared, for example, from an alkoxide of titanium and an alcohol amine; coating the mixture onto a substrate; and then heating the coated substrate gradually from room temperature to a temperature of 600 to 700xc2x0 C. to prepare a thin film of an anatase titanium oxide. In this publication, there is a description to the effect that the baking temperature is preferably 600 to 700xc2x0 C. This indicates that the step of sintering is necessary for the production of anatase titanium oxide.
Japanese Patent Laid-Open No. 277147/1996 also proposes a coating material produced by the sol-gel process. In this case, the step of baking at 350xc2x0 C. is provided. Japanese Patent Laid-Open No. 21557/1996 also proposes the use, as a coating material, of a titanium oxide sol, prepared by hydrolysis of a titanyl sulfate, after dilution with water. Also in this case, baking at 300xc2x0 C. in the air is carried out.
Japanese Patent Laid-Open No. 257360/1996 proposes a production process which comprises the steps of: dispersing previously prepared anatase titanium oxide powder (F-25, manufactured by Nippon Aerosil Co., Ltd.) together with finely divided cellulose in water; and adding polyaluminum chloride as a coagulant to prepare a slurry material.
Regarding the dispersion of powdery anatase titanium oxide into water, for example, there is a report such that metatitanic acid is prepared from ilmenite as a starting material by the sulfuric acid process, and nitric acid is added to the metatitanic acid, followed by the dispersion of titanium oxide in the mixture to improve the dispersion and storage stability of the coating material (Kogyo Zairyo Vol. 45, No. 10, p. 48 (1997)).
In the production of anatase titanium oxide powder by the gas phase process, all the above production processes have drawbacks such as the necessity of using a special apparatus in the production of anatase titanium oxide due to the adoption of high-temperature reaction atmosphere (generally 800xc2x0 C. or above in the case of the gas phase process) and the use of highly reactive titanium chloride as a starting material. In this connection, it should be noted that the above-described production processes of anatase titanium oxide by the gas phase process are a dry process which is utterly different from the production process of anatase titanium oxide by the liquid phase process according to the present invention.
Also in the case where the anatase titanium oxide powder is produced by the sol-gel process, as described above, the step of sintering is indispensable, and, to this end, heat treatment at a temperature of 300xc2x0 C. or above is necessary.
In HyCOM, as described above, highly heat-resistant anatase titanium oxide is obtained. In this process, however, a special apparatus should be disadvantageously used for the preparation of anatase titanium oxide. In HyCOM, a solvent with an alkoxide being dissolved therein and water are placed separately from each other in the apparatus, and, thus, water is absent in the starting material. By contrast, according to the present invention, water is previously added to a titanium alkoxide as the starting material to hydrolyze the titanium alkoxide to titanium hydroxide. Thus, the HyCOM and the production process according to the present invention are utterly different from each other, for example, in procedure.
The production of anatase titanium oxide powder by the sulfuric acid process is disadvantageous in that the number of steps is large and, for example, the operation is very complicate.
For the anatase titanium oxide-containing slurry, in the case of a coating material prepared by hydrolysis, that is, a coating slurry, the step of heat treatment at 300xc2x0 C. or above is necessary to finally prepare anatase titanium oxide. Due to this high temperature, the coating material cannot be coated on base materials having low heat resistance. Therefore, usable base materials are limited.
In the preparation of the coating material, dispersing powdery anatase titanium oxide in a solvent causes the coagulation of titanium oxide particles which makes it impossible to maintain the activity of the particulate photocatalyst. Further, in the form of the coating material, for example, the dispersion of anatase titanium oxide in the solution in heterogeneous, and, with the elapse of the time, titanium oxide particles disadvantageously settle at the bottom of the solution. That is, the coating material has a problem of storage stability. As with the above technique, the problem of particle coagulation or the like occurs also in the method where metatitanic acid is prepared from ilmenite as a starting material by the sulfuric acid process, and nitric acid is added to the metatitanic acid, followed by the dispersion of titanium oxide in the mixture.
Further, a method, wherein a third component in addition to titanium oxide is added for improving the adhesion, and a method, wherein a precoating is prepared, are also known.
For example, Japanese Patent Laid-Open No. 131834/1996 describes a method wherein a thermoplastic binder, such as acrylic resin, or a thermosetting resin, such as fluororesin, epoxy resin, or cyloxane resin, is formed on the surface of the substrate.
A method is also proposed wherein PTFE as one of fluororesins is used as a binder (xe2x80x9cHikarishokubai No Sekai (The world of photocatalysts),xe2x80x9d issued in April 1998 by Kogyo Chosakai Publishing Co., Ltd.).
A proposal has also been made wherein photocatalytic decomposition-free silica is precoated on a substrate and, thereafter, a titanium oxide layer is coated, thereby improving the adhesion strength.
Thus, regarding the binder for ensuring the strength of adhesion to the member, the addition of ceramics, such as silica (SiO2), or polymers, such as polymethyl methacrylate (PMMA), has hitherto been made.
However, when the adhesion of anatase titanium oxide onto the surface of the member with high adhesion strength is contemplated, as described in Japanese Patent Laid-Open Nos. 99041-1996 and 277147/1996, there is a problem such that treatment at a high temperature of 300xc2x0 C. or above is necessary and, thus, titanium oxide cannot be coated onto members having low heat resistance.
The conventional binder user f r improving the adhesion of titanium oxide has the following problems.
As described in Japanese Patent Laid-Open No. 131834/1996, when organic matter is mixed, the photocatalyst decomposes the organic matter and thus cannot maintain the adhesion strength. Therefore, there is a limitation on the long-term persistence of the photocatalytic activity. Likewise, in the case of polymers, such as polymethyl methacrylate (PMMA), the decomposition of PMMA takes place by the photocatalytic activity, leading to a limitation on the persistence of adhesion strength.
In the case of silica, although the adhesion strength can be maintained, a separate material is necessary. This deteriorates the photocatalytic activity.
Japanese Patent Laid-Open No. 67516/1998 describes a method which comprises preparing a titanium hydroxide precipitate from a titanium solution and a basic solution, adding thereto aqueous hydrogen peroxide, and then heat treating the mixture at a temperature of 80xc2x0 C. or above. By contrast, the production process according to the present invention is utterly different from this method in that an organic solvent is used, the step of recovering precipitate is not provided, and ozone treatment is carried out at a temperature of about 25xc2x0 C. or below.
Further, in the prior art technique, as described in the above publication, there are two coating steps which render the process complicate.
An object of the present invention is to provide a process for producing anatase titanium oxide powder wherein a titania sol, a titania gel, or a titania sol-gel mixture is heat treated in a closed vessel to hybridize the effect of pressure, whereby anatase titanium oxide powder having high photocatalytic activity and large specific surface area can be produced at a low heat treatment temperature of 250xc2x0 C. or below via a small number of steps in a simple manner.
Another object of the first invention is to provide a process for producing anatase titanium oxide slurry wherein anatase titanium oxide produced by heat treatment within a closed vessel under pressure is subjected to, for example, ultrasonic disperion r stirring in the solvent used in the preparation of a titania sol, a titania gel, or a titania sol-gel mixture, whereby anatase titanium oxide slurry, which is very stable, is free from settling of titanium oxide particles, and can also be coated on the surface of materials having low heat resistance, can be produced at room temperature.
In order to attain the above object, the process for producing anatase titanium oxide according to the first invention comprises the steps of: heat treating a titania sol solution, a titania gel, or a titania sol-gel mixture in a closed vessel under pressure, said titania sol solution, titania gel, or titania sol-gel mixture containing as a solvent an alcohol represented by structural formula CnH2n+2OH; and then drying the treated product to prepare anatase titanium oxide powder.
The process for producing anatase titanium oxide according to the first invention comprises the steps of: heat treating a titania sol solution, a titania gel, or a titania sol-gel mixture in a closed vessel under pressure, said titania sol solution, titania gel, or titania sol-gel mixture containing as a solvent an alcohol represented by structural formula CnH2n+1OH; and then ultrasonically dispersing or stirring the treated product to prepare anatase titanium oxide slurry. As described above, stirring may be ultrasonic dispersion and, in addition, may of course be mechanical agitation or the like.
In these cases, examples of starting materials usable for the production of a titania sol or a titania gel include metal organic compounds, e.g., metal alkoxides and titanium oxalate, and metal inorganic compounds, e.g., titanium nitrate and titanium tetrachloride. Metal alkoxides include, for example, titanium tetramethoxide, titanium tetraethoxide, titanium isopropoxide, and titanium tetrabutoxide.
In the production process according to the present invention, the titania sol solution, the titania gel, or the titania sol-gel mixture is preferably heat treated in a closed vessel in the temperature range of 80 to 250xc2x0 C.
As described above, the temperature, at which the contents of the closed vessel are heated, should be 80 to 250xc2x0 C. because the solvent, which has dissolved the starting material, should be evaporated. When the heat treatment temperature is below 80xc2x0 C., a lot of time is necessary for evaporating the whole solvent and, thus, satisfactory pressure cannot be applied. Further, in this case, the amount of anatase titanium oxide produced is not very large even after treatment for a considerably long period of time while applying pressure, and, thus, the treatment at a temperature below 80xc2x0 C. is not suitable from the practical viewpoint. On the other hand, the treatment at a temperature above 250xc2x0 C. requires special structure and equipment such as closed vessels and sealing materials.
Further, in the production process according to the present invention, the titania sol solution, the titania gel, or the titania sol-gel mixture is preferably treated in a closed vessel at a pressure of 1.5 to 350 atmA, more preferably 5 to 60 atmA.
For the pressure of treatment within the closed vessel, as described above, the lower limit is preferably 1.5 atmA, more preferably 5 atmA, while the upper limit is preferably 350 atmA, more preferably 60 atmA. When the pressure of treatment within the closed vessel is below the above lower limit, the dispersibility of the resultant anatase titanium oxide slurry is deteriorated. On the other hand, when the pressure of treatment within the closed vessel exceeds the above upper limit, extra apparatuses and equipment for the application of pressure should be additionally provided.
Further, in the production process according to the present invention, preferably, the contents of the closed vessel are heated to evaporate the solvent contained in the titania sol solution, the titania gel, or the titania sol-gel mixture, whereby the inside of the closed vessel is pressurized by gas generated as a result of the evaporation of the solvent. In this case, the pressure within the closed vessel can be controlled by the volume and the amount of the solvent within the closed vessel, and this can bring the pressure within the closed vessel to the above treatment pressure in the above defined range.
Further, the introduction of pr ssurized inert gas into the closed vessel can also regulate the pressure within the closed vessel.
As described above, according to the production process of the present invention, an alcohol having a structure represented by formula CnH2n+1OH is contained as a solvent in the titania sol solution, the titania gel, or the titania sol-gel mixture.
The solvent for dissolving the starting material may be any solvent which can dissolve the titania sol or the titania gel. In particular, an alcohol having a structure represented by formula CnH2n+1OH is preferably used as the solvent. For example, methanol, ethanol, 1-propanol, isopropyl alcohol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol, 2-pentanol, and 3-pentanol are preferably used.
Further, according to the production processes of the present invention, preferably, at least one member selected from the group consisting of acidic materials, alkaline materials, organic polymers, and inorganic materials is added to the titania sol solution, the titania gal, or the titania sol-gel mixture.
If necessary, acidic materials, such as hydrochloric acid, nitric acid, and acetic acid, alkaline materials, such as ammonia and amine compounds, inorganic materials, such as silica, polymethacrylic acid resins, fluororesins, aromatic organic polymers and the like may be added to the starting material, such as titania sol.
In the first invention, a titania sol, prepared, for example, by adding a minor amount of water to titanium isopropoxide in isopropyl alcohol as an organic solvent, is heat treated in a closed vessel to evaporate isopropyl alcohol and to promote the crystallization to form anatase titanium oxide utilizing the vapor pressure and heat. In this case, the resultant titanium oxide slurry contains a large amount of isopropyl alcohol.
Japanese Patent Laid-Open No. 71418/1997 discloses a techniqu wherein a titanium hydroxide precipitate is first prepared from a titanium solution and a basic solution, such as an ammonia solution, and aqueous hydrogen peroxide is then added, followed by heat treatment at 80xc2x0 C. or above to crystallize anatase titanium oxide. In this publication, although autoclaving is disclosed, there is no description on detailed treatment conditions and the like. According to the present invention, for example, a metal alkoxide is used as the starting material, and, when the metal alkoxide is mixed with aqueous hydrogen peroxide, hydrolysis and dissolution are simultaneously carried out without the formation of any precipitate, followed by heat treatment of the reaction solution at a temperature of 120 to 270xc2x0 C., preferably 150 to 240xc2x0 C., under hermetically sealed conditions to efficiently crystallize titanium oxide in a short time. Thus, the present invention is substantially different from the technique described in the above publication.
In the first invention wherein a titania sol or the like prepared by using an organic solvent as a solvent is used, coatability onto members having low organic solvent resistance is not always satisfactory. Further, when the organic solvent is used, there are many metal salts insoluble in the organic solvent. Therefore, disadvantageously, suitable metal salts cannot be added to titania sols or the like.
Accordingly, an object of the second invention is to provide production processes of anatase titanium oxide powder oxide powder and aqueous anatase titanium oxide slurry, wherein a substantially organic solvent-free, aqueous titania sol, titania gel, or titania sol-gel mixture is heat treated in a closed vessel under pressure, whereby anatase titanium oxide powder having high photocatalytic activity and large specific surface area and aqueous anatase titanium oxide slurry can be produced at a low heat treatment temperature of 270xc2x0 C. or below, preferably 240xc2x0 C. or below, without the generation of any organic solvent at the time of drying via a small number of steps in a simple manner.
Another object of the second invention is to provide production processes of powder composed mainly of anatase titanium oxide capable of developing photocatalytic activity and aqueous titanium xide slurry containing the same, which involve the step of hydrolyzing an titanium alkoxide with water contained in aqueous hydrogen peroxide or aqueous ozone to produce amorphous titanium oxide and, at the same time, instantaneously dissolving the produced amorphous titanium oxide in aqueous hydrogen peroxide or aqueous ozone, and the step of heat treating the resultant aqueous titania sol, titania gel, or titania sol-gel mixture in a closed vessel under pressure.
In order to attain the above object, the process for producing anatase titanium oxide according to the second invention comprises the steps of: heat treating an organic solvent-free aqueous titania sol solution, titania gel, or titania sol-gel mixture in a closed vessel under pressure; and then drying the treated product to prepare anatase titanium oxide powder.
Further, the process for producing anatase titanium oxide according to the second invention comprises the steps of: heat treating a substantially organic solvent-free aqueous titania sol solution, titania gel, or titania sol-gel mixture in a closed vessel under pressure; and then stirring or ultrasonically dispersing the treated product to prepare aqueous anatase titanium oxide slurry. As described above, for example, mechanical agitation and ultrasonic dispersion may be used for stirring.
In the production process according to the present invention, the organic solvent-free aqueous titania sol solution, titania gel, or titania sol-gel mixture is preferably heat treated in the temperature range of 120 to 270xc2x0 C., more preferably in the temperature range of 150 to 240xc2x0 C., in the closed vessel.
As described above, the temperature, at which the contents of the closed vessel are heated, is preferably 120 to 270xc2x0 C., more preferably 150 to 240xc2x0 C., because the solvent (water), which has dissolved the starting material, should be vaporized in a closed vessel. When the heating temperature is below the lower limit of the above range, the vaporization of water is unsatisfactory and, thus, the application of pressure is unsatisfactory. Therefore, in this case, the crystallization to form anatase titanium oxide cannot be promoted. On the other hand, in the case of heat treatment at a temperature above the upper limit of the above temperature range, the crystallization proceeds to an excessive extent. In this case, titanium oxide having large particle diameters is formed, and, thus, the dispersibility in water is poor.
In the production process according to the present invention, preferably, the contents of the closed vessel are heated to evaporate the solvent contained in the organic solvent-free aqueous titania sol solution, titania gel, or titania sol-gel mixture, whereby the inside of the closed vessel is pressurized at a pressure of 1.5 to 33 atmA by gas generated as a result of the evaporation of the solvent.
The pressure within the closed vessel can be controlled by the volume and the amount of the solvent within the closed vessel, and this can bring the pressure within the closed vessel to the above defined treatment pressure range. As described above, the treatment pressure should fall within the range of 1.5 to 33 atmA. When the pressure within the closed vessel is below the lower limit of the above pressure range, a lot of time is necessary for crystallization to form anatase titanium oxide. On the other hand, when the pressure within the closed vessel exceeds the upper limit of the above pressure range, special structure and equipment, such as closed vessels and sealing materials, are necessary and, in addition, extra apparatuses and equipment are necessary for the application of pressure.
Further, the introduction of pressurized insert gas into the closed vessel can also regulate the pressure within the closed vessel.
In these production processes according to the present invention, preferably, a titanium alkoxide is provided as a starting material for the production of an organic solvent-free aqueous titania sol solution, titania gel, or titania sol-gel mixture and is hydrolyzed in aqueous hydrogen peroxide or aqueous ozone and, at the same time, is dissolved in aqueous hydrogen peroxide or aqueous ozone to produce an organic solvent-free aqueous titania sol solution, titania gel, or titania sol-gel mixture.
As described above, for example, titanium alkoxides as metal organic compounds may be used as the starting material for the production of a titania sol or a titania gel. Titanium alkoxides include, for example, titanium tetramethoxide, titanium tetraethoxide, titanium isopropoxide, titanium n-propoxide, titanium tetra n-butoxide, titanium tetraisobutoxide, titanium methoxypropoxide, and titanium dichloride diethoxide.
The aqueous solvent for dissolving and hydrolyzing the starting material may be any solvent so far as the solvent is water containing a peroxide. Particularly preferred are aqueous hydrogen peroxide and aqueous ozone.
Further, in these production processes according to the present invention, preferably, at least one member selected from the group consisting of water-soluble metal salts, acidic materials, alkaline materials, organic polymers, inorganic materials, and metal alkoxides other than titanium is added to the organic solvent-free aqueous titania sol solution, titania gel, or titania sol-gel mixture.
A single water-soluble metal salt compound or a plurality of water-soluble metal salt components may be added to the starting material such as titania sol. Metal salts include acetates, nitrates, oxalates, sulfates, and chlorides. Metals contained in the metal salts include platinum, gold, silver copper, sodium, magnesium, aluminum, potassium, calcium, vanadium, chromium, manganese, cobalt, nickel, zinc, selenium, zirconium, molybdenum, palladium, tin, hafnium, and tungsten.
If necessary, acidic materials, such as hydrochloric acid, nitric acid, and acetic acid, alkaline materials, such as ammonia and amine compounds, inorganic materials, such as silica, polymethacrylic acid resins, fluororesins, organic polymers, such as aromatic organic polymers, and metal alkoxides other than titanium alkoxide may be added to the starting material, such as titania sol. Other metal alkoxides include alkoxides of aluminum, antimony, barium, bismuth, boron, calcium, cerium, cesium, chromium, copper, gallium, hafnium, iron, lithium, lutetium, magnesium, molybdenum, niobium, nickel, palladium, platinum, rhodium, samarium, silicon, silver, tungsten, vanadium, yttrium, zinc, zirconium, and th like. Further, metal akoxides may be those wherein a single or a plurality f metals may be contained in the metal alkoxides.
This technique is not limited to the production of titanium oxide, and can also be applied to the production of photocatalysts such as zinc oxide.
An object of the third invention is to provide a process for producing a titanium oxide coating material wherein, for example, a liquid prepared by treating a titania sol solution, a titania gel, or a titania sol-gel mixture with ozone gas is used to produce a titanium oxide coating material which can realize coating of titanium oxide having excellent photocatalytic activity at a low temperature, can be coated onto members with high adhesion strength, which can be maintained for a long period of time, and has excellent transparency which does not deteriorate the appearance of members. Another object of the third invention is to provide a process for producing a titanium oxide coating material wherein a liquid prepared by treating a titania sol solution, a titania gel, or a titania sol-gel mixture with ozone gas is mixed with titania powder or titania slurry to produce a binder that can maintain adhesion to materials having low adhesion strength for a long period of time although the photocatalytic activity is high.
In order to attain the above object, the process for producing titanium oxide coating material according to the third invention comprises the steps of: providing a titania sol solution, a titania gel, or a titania sol-gel mixture; and treating the titania sol solution, the titania gel, or the titania sol-gel mixture with ozone gas to produce a titanium oxide coating material.
Further, according to the production process of a titanium oxide coating material according to the present invention, the titania sol solution, the titania gel, or the titania sol-gel mixture may be treated with ozone gas, followed by mixing of the treated sol, gel, or sol-gel mixture with titania powder or titania slurry (at least one of titanium oxide powder, titanium oxide slurry, and a mixture thereof) to produce a titanium oxide coating material.
In these cases, for example, titanium alkoxides and titanium oxalate as metal organic compounds and titanium nitrate and titanium tetrachloride as metal inorganic compounds may be used as starting materials for the production of titania sol or titania gel. Titanium alkoxides include, for example, titanium tetramethoxide, titanium tetraethoxide, titanium isopropoxide, titanium n-propoxide, titanium tetra-n-butoxide, titanium tetraisobutoxide, titanium methoxypropoxide, and titanium dichloride diethoxide.
In these production processes according to the present invention, water, an organic solvent, or a mixture thereof, preferably, an alcohol may be contained as a solvent in the titania sol solution, the titania gel, or the titania sol-gel mixture.
The solvent for dissolving the starting material may be any organic solvent or water so far as the solvent can dissolve titania sol or titania gel. Organic solvents include alcohols, hydrocarbons, halogenated hydrocarbons, phenols, ethers, esters, and nitrogen compounds. More specific examples of organic solvents include hexane, toluene, benzene, xylene, cyclohexane, carbon tetrachloride, phenol, cresols, diethyl ether, dioxane, acetone, ethyl acetate, propyl acetate, acetonitrile, tetrahydrofuran, and pyridine. In particular, the use of alcohols having a structure represented by formula CnH2n+1OH is preferred. These alcohols include, for example, methanol, ethanol, 1-propanol, isopropyl alcohol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol, 2-pentanol, and 3-pentanol.
In the above production process according to the present invention, the starting material, for a titanium oxide coating material, after ozone treatment may be used after dilution with an organic solvent, preferably an alcohol. The alcohol used for the dilution is preferably an alcohol having a structure represented by formula CnH2n+1OH, particularly preferably isopropyl alcohol. Dilution with an organic solvent other than alcohols or a mixed solution composed of a plurality of organic solvents is als possibl . The dilution of the ozone-treated coating material, for example, with an alcohol is carried out from the viewpoints of improving the concentration of the coating material and the effect of inhibiting the gelation.
Further, in these production processes according to the present invention, at least one member selected from the group consisting of acidic materials, alkaline materials, surfactants, and metal alkoxides other than titanium alkoxide is preferably added to the titania sol solution, the titania gel, or the titania sol-gel mixture.
Thus, if necessary, acidic materials, such as hydrochloric acid, nitric acid, and acetic acid, alkaline materials, such as ammonia and amine compounds, surfactants, and metal alkoxides other than titanium alkoxides may be added to the starting material, such as titania sol.
Other metal alkoxides include alkoxides of aluminum, antimony, barium, bismuth, boron, calcium, cerium, cesium, chromium, copper, gallium, hafnium, iron, lithium, lutetium, magnesium, molybdenum, niobium, nickel, palladium, platinum, rhodium, samarium, silicon, silver, tungsten, vanadium, yttrium, zinc, zirconium and the like. Further, metal akoxides may be those wherein a single or a plurality of metals may be contained in the metal alkoxides.